Anatomical location and morphology
Merkel cells are located in the basal epidermis of both glabrous and hairy skin of mammals. In glabrous skin, clusters of 4-40 Merkel cells are present in the epidermal pegs, which are protrusions of the epidermis into the dermis that surrounds the dermal papillae. In primates and marsupials, smaller clusters of Merkel cells are located at the base of the epidermal ridges which are responsible for the fingerprint pattern of the hands and feet (Halata et al., 2003). These clusters of Merkel cells in glabrous skin are often referred to as “touch spots” (Boulais and Misery, 2007). In hairy skin, Merkel cells are present in “touch domes”, which can be discerned by a slight elevation in the skin in some species, and can contain up to 150 Merkel cells. Touch domes may or may not be associated with a hair follicle (Zelena, 1994). In rodents, Merkel cells are closely associated with guard hair follicles, located in the epidermis in “collars” surrounding the
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hair follicle, and whiskers, located in “cuffs” present underneath the glassy membrane of the follicle (Halata and Munger, 1980b; Zelena, 1994).
Merkel cells were first described as “touch cells” by Freidrich Sigmund Merkel in 1875 (Merkel, 1875). They are oval in shape and 10-15 μm in length along the long axis, which is the smallest among all mechanosensory endings discussed in this introduction. They can be differentiated from the surrounding epidermal cells by their large,
multilobated nucleus which is oriented parallel to the dermis-epidermis junction. On the basal side of the cell, numerous dense core vesicles measuring 70-180 nm in diameter are located close to the cell membrane (Halata et al., 2003; Iggo and Muir, 1969; Tachibana and Nawa, 2002; Winkelmann and Breathnach, 1973). Most Merkel cells are associated with an innervating axon, forming a structure referred to as the Merkel cell-neurite complex. A nerve plate, which is formed by a myelinated axon which loses its myelin sheath upon entering the epidermis, directly opposes the vesicle dense basal membrane of the Merkel cell. This plate is separated from the Merkel cell by 15 nm, but in small regions they are separated by only 13 nm, and electron dense material is observed in both the Merkel cell and the nerve plate at these points (Halata et al., 2003; Iggo and Muir, 1969). A small number of Merkel cells do not make contact with an innervating axon. The function of these uninnervated Merkel cells is unknown, but it has been suggested they may have neuroendocrine or immune system functions (Boulais and Misery, 2007).
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Central projections of Merkel cell innervating neurons bifurcate upon entering the spinal cord, and send collaterals into the dorsal horn as these branches travel anteriorly and posteriorly. The morphology of the collaterals innervating the dorsal horn are distinct form the RA mechanoreceptors. Individual collaterals dive into the dorsal horn
perpendicular to the dorsal surface of the spinal cord. After reaching layer IV or V, the collaterals make a C- or L-shaped turn and then travel medially. During and after this turn, the collaterals gives off terminal arborizations in layers III-V (Brown, 1981).
Physiological properties and function
Merkel cell-neurite complexes are type I SA (SAI) mechanoreceptors. Unlike the RA mechanoreceptors discussed above, SA mechanoreceptors remain active during the static phase of stimuli. The innervating neuron is usually silent at rest, and responds to the onset of stimulation with a burst of activity, which is proportional to the velocity and displacement of the stimulus. After the initial phasic burst of activity, a tonic firing phase occurs for the duration of the application of the stimulus. The firing pattern during the tonic phase is irregular and can last for over 30 minutes (Iggo and Muir, 1969; Tapper, 1965; Willis and Coggeshall, 2004).
There has been a long-standing debate with regard to the exact role of Merkel cells in light touch sensation. Many studies suggested that the Merkel cell is critical for transducing the mechanical stimulus into a chemical signal to activate the innervating
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neurite, while other studies proposed that Merkel cells may play a modulatory role and the neurite is primarily responsible for transducing the mechanical stimulus.
There is abundant biochemical evidence that Merkel cells produce neurotransmitters and the machinery required for synaptic release (Haeberle et al., 2004; Maksimovic et al., 2013; Tachibana and Nawa, 2002). Merkel cells also express voltage gated calcium channels, and calcium induced calcium release from internal stores occurs upon the entry of calcium into the cell, providing a potential mechanism for neurotransmitter release (Senok and Baumann, 1997; Yamashita et al., 1992). In addition, blocking glutamatergic transmission reduced the SA response evoked by activation of Merkel cell-neurite
complexes, suggesting that excitatory neurotransmission is required for transducing the mechanical stimulus (Fagan and Cahusac, 2001). The most compelling evidence suggesting a mechanosensory function of Merkel cells comes from Atoh1/Math1 conditional knockout mice, in which Merkel cells are not differentiated but the
innervating fibers are still present in touch domes. Strikingly, although the total number of Aβ fibers is not significant changed, SAI responses could not be detected in these animals using the ex vivo skin nerve preparation (Maricich et al., 2009). These results suggest that Merkel cells are essential for mediating the SAI response. However, it is unclear whether these remaining fibers completely lost their mechanosensitivity or display physiological properties similar to RA mechanoreceptors.
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On the other hand, using both ultrastructural and electrophysiological evidence,
Gottschaldt and Vahle-Hinz argued that the ability of Merkel cell innervating neurites to follow high frequency stimuli up to 1200 Hz with a one-to-one response for up to 500 ms is incompatible with chemical communication, as neurotransmitter could not be released and cleared from the synapse quickly enough to produce such a precise response. In addition, the latency from application of stimulus to response in the innervating fiber was too fast for chemical transmission (Gottschaldt and Vahle-Hinz, 1981), further supporting their model that the innervating neurite acts as the mechanosensitive element.
Diamond’s group also argued that Merkel cells are dispensable for mechanosensation. They found that touch domes were still mechanoresponsive after selective destruction of Merkel cells using quinacrine loading and ultraviolet (UV) light irradiation (Diamond et al., 1988). However, Ikeda et al. found that SAI responses evoked by touch dome stimulation were lost using a different irradiation procedure to eliminate Merkel cells (Ikeda et al., 1994). Further investigation revealed that quinacrine loading/UV irradiation is not selective and incomplete, leaving some Merkel cells relatively intact and damaging other nerve fibers in the skin (Senok et al., 1996). These conflicting results and technical issues make these experiments difficult to interpret.
Others have attempted to reconcile the conflicting findings regarding the role of the Merkel cell in mechanotransduction with a two-receptor-site model, in which both the innervating neurite and the Merkel cell are mechanosensitive. According to this model, the early phasic activity is mediated by the neurite while the late tonic phase is due to
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chemical communication between the Merkel cell and the neurite (Maksimovic et al., 2013; Ogawa, 1996). A key to resolving this debate is to develop new tools, by which Merkel cell innervating axons can be specifically identified for physiological recording while Merkel cells are acutely and selectively ablated.
As a result of their small sizes, Merkel cell-neurite complexes have the smallest receptive fields among all mechanoreceptors and are best able to distinguish individual closely spaced objects. Due to these characteristics, Merkel cells are proposed to be essential for detecting the fine details of touched objects, such as shape, texture, and curvature
(Johnson, 2001; Johnson et al., 2000). A recent study found that mice lacking Merkel cells were unable to detect certain textures with their feet (Maricich et al., 2012).
Development
In contrast to the RA mechanoreceptor end organs, Merkel cells appear in the skin prior to the arrival of innervating fibers (Saxod, 1996). Immature Merkel cells can be observed in the epidermis of the rat around E16, when innervating axons have only reached the dermis. Axons reach the epidermis and are found in close association with Merkel cells a day later (English et al., 1980).
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Since Merkel cells share characteristics with both epidermal and neural cells, there has been argument concerning the embryological origins of Merkel cells in mammals (Lucarz and Brand, 2007). In birds, chick-quail chimera experiments offer strong support to a neural crest origin of Merkel cells (Grim and Halata, 2000). In addition, Merkel cells were labeled by LacZ when all neural crest derived cells were genetically labeled using Wnt1Cre and ROSA26R-β-Galactosidase reporter mice, suggesting that mouse Merkel cells arose from a neural crest origin (Szeder et al., 2003). However, there is also genetic evidence to support an epidermal origin of Merkel cells. The transcription factor
Atoh1/Math1 is highly expressed and functionally required for the development of Merkel cells. Surprisingly, Merkel cells still form when Atoh1 is conditionally ablated from neural crest cells. In contrast, when Atoh1 is deleted from the basal layer of the epidermis using a Keratin14Creline, Merkel cells do not form, suggesting an epidermal
origin for Merkel cells in mammals (Morrison et al., 2009). Nevertheless, this study could not exclude a cell non-autonomous effect for Atoh1 in the development of Merkel cells, as Atoh1 expression was also observed in some of the accessory cells surrounding Merkel cells.
Inherent transcriptional programs in the innervating neuron are also required for development of Merkel cell-neurite complex. Similar to Meissner’s corpuscles and lanceolate endings, Merkel cells are dependent on the transcription factor Shox2for sensory innervation during development. In Shox2 mutants, Merkel cells are present in both glabrous and hairy skin, but there is a dramatic decrease in the percentage of Merkel
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cells innervated by large diameter sensory fibers (Abdo et al., 2011). In addition, some mechanoreceptive DRG neurons co-express the Runt-related transcription factors Runx1 and Runx3 (Yoshikawa et al., 2013), and the number of Merkel cell-neurite complex surrounding the whiskers is greatly reduced in Runx3 mutant mice (Senzaki et al., 2010). One plausible mechanism by which Runx3 controls Merkel cell-neurite complex
development is to regulate TrkC expression (Kramer et al., 2006b; Levanon et al., 2002; Nakamura et al., 2008).
In addition to transcriptional programs, Merkel cell-neurite complexes are highly
dependent on several types of neurotrophic signaling for their development (Montano et al., 2010). One population of Merkel cells depends on TrkA/NGF signaling. The
number of Merkel cells surrounding hair follicles and the number of innervating axons is reduced in TrkA mutants. However, remaining Merkel cells are maintained into
adulthood, suggesting a TrkA independent Merkel cell population. Loss of the TrkA ligand NGF produces a similar, but less severe, phenotype (Fundin et al., 1997).
TrkC/NT3 signaling has a significant and complicated effect on Merkel cell development. TrkC is expressed in both Merkel cells and Merkel cell innervating somatosensory
neurons. In mice lacking the kinase domain of TrkC, the number of Merkel cells is reduced at birth. In addition, those that are present at birth are not maintained during the first two postnatal weeks, suggesting that all Merkel cells become dependent on TrkC
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signaling postnatally (Cronk, Wilkinson et al. 2002). Loss of NT3, the TrkC ligand, leads to a more severe deficit, with even fewer Merkel cells present at birth (Airaksinen et al., 1996). The phenotype becomes most severe when all isoforms of TrkC are
eliminated in TrkC complete null mice, in which no Merkel cells or innervating fibers are present at birth, suggesting additional kinase-independent roles of TrkC in Merkel cell development (Cronk et al., 2002; Fundin et al., 1997).
TrkB signaling also has an effect on Merkel cell development. In TrkB mutant mice, the number of Merkel cells surrounding hair follicles and in the glabrous skin is greatly reduced (Perez-Pinera et al., 2008). When BDNF is overexpressed in the skin, the number of Merkel cells is increased in the glabrous but not hairy skin (LeMaster et al., 1999). Interestingly, the mechanical threshold of SAI mechanoreceptors increases eight fold in the BDNF heterozygous and null mice, although the number and morphology of Merkel cells was normal in touch domes of P14 BDNF null animals. This deficit could be rescued by injecting recombinant BDNF into BDNF heterozygous mice (Carroll et al., 1998).
Lastly, the low affinity neurotrophin receptor p75 also plays a role in Merkel cell development. p75 can bind NGF, BDNF, NT3, and NT4, and interact with the Trk receptors (Skaper, 2012). In p75 mutant mice, Merkel cells develop normally during the first two postnatal weeks but then then slowly decrease in number over the following months until very few remain (Fundin et al., 1997; Kinkelin et al., 1999).
29 RUFFINI CORPUSCLES
Anatomical location and morphology
The Ruffini corpuscle is an elongated structure with tapered ends. Morphologically, it is quite similar to the Golgi tendon organs which are innervated by proprioceptors (Halata and Munger, 1980a). The corpuscle is usually encased in a capsule of 4-5 layers of perineural cells and contains an inner core of Schwann cells and collagen, which is innervated by a single large diameter myelinated axon that loses its myelination upon entry into the inner core. The axon gives off numerous terminal branches within the inner core (Chambers et al., 1972; Willis and Coggeshall, 2004). Collagen fibers associated with the inner core exit the poles of the Ruffini corpuscle and interact with collagen in the surrounding tissue, providing a potential mechanism for mechanically linking the inner core with the surrounding tissue (Halata, 1977).
The central projections of SA type II (SAII) mechanoreceptors, which are presumed to innervate Ruffini corpuscles, are distinct form other mechanoreceptors. Collaterals innervating the dorsal horn project to layer III, and then branch into at least two processes. These processes travel deeper into the dorsal horn and branch extensively, forming terminal arborizations from layer III-VI (Brown, 1981).
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The first extensive morphological characterization of Ruffini corpuscles was performed on hairy skin of the cat (Chambers et al., 1972). However, the anatomical location and existence of Ruffini corpuscles between tissues and species is currently under debate. In many cases, numerous units with SAII responses can be recorded in nerve fibers
innervating a tissue, but the Ruffini corpuscles cannot be found in the tissue following careful histological examination. For example, physiological recordings of nerves innervating the glabrous skin of raccoons and humans have shown a relatively high proportion of units exhibiting SAII responses (Johansson and Vallbo, 1979; Rasmusson and Turnbull, 1986). However, when glabrous skin from monkeys and raccoons was examined, no Ruffini corpuscles were found (Pare et al., 2002; Rice and Rasmusson, 2000). In humans, a single Ruffini corpuscle was found in the skin of the index finger, which is much less than what would be expected based on the physiological recordings (Pare et al., 2003). Notably, Pare, et al. observed innervation of blood vessels which looked morphologically similar to previous descriptions of Ruffini corpuscles. The authors suggest that previous studies may have misidentified these structures as Ruffini corpuscles, which could explain the discrepancy in previous findings (Pare et al., 2002). In mouse hairy skin, SAII fibers are also identified by physiological recordings but no definite Ruffini corpuscle structure has been reported (Wellnitz et al., 2010).
In many species, sensory endings which are morphologically similar to Ruffini
corpuscles have been identified. In monkeys and raccoons, unencapsulated Ruffini-like endings were found at the base of the fingernail/claw (Pare et al., 2002; Rice and
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Rasmusson, 2000). Ruffini corpuscles have also been found in association with hair follicles, where they are sometimes referred to as pilo-Ruffini complexes (Biemesderfer et al., 1978). Additionally, unencapsulated periodontal Ruffini-like corpuscles have been identified surrounding the teeth of rodents (Byers, 1985).
Physiology and function
Although not well defined morphologically, the physiology of SAII Aβ low-threshold mechanoreceptors have been extensively characterized in both humans and model organisms (Johansson and Vallbo, 1979; Wellnitz et al., 2010). Like the SAI response, the SAII responses is characterized by an early dynamic phase which is sensitive to both the velocity and displacement of the stimulus, followed by a static response phase that last throughout the application of stimulus. However, SAII responses can be
differentiated from SAI responses because they usually display some background firing activity when no stimulus is applied, they fire at a much more regular rate during the static phase, and their maximum frequency of the response is less than that of the SAI response (Chambers et al., 1972).
SAII Aβ low-threshold mechanoreceptors are proposed to act primarily as stretch
receptors. In psychophysical recordings SAII units were less sensitive than SAI units to skin indentation, but were much more sensitive to stretching of the skin (Johnson et al., 2000). The stretch receptors in the skin may have two functions. They may work in
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combination with RA mechanoreceptors to sense movement of grasped objects. In addition, they may work in concert with proprioceptors to sense the position of the fingers and hand, as skin stretch will vary based on grasp (Johnson, 2001).
Development
Due to the difficulty in clearly identifying Ruffini corpuscles by morphology, relatively little work has been done to study their development compared to the other
mechanoreceptors discussed in this review. Both periodontal Ruffini-like endings and those associated with whisker hair are dependent on TrkB neurotrophic signaling in mice. Periodontal Ruffini-like endings are absent in TrkB mutant mice, and mice lacking either BDNF or NT4 show Ruffini-like endings with immature morphology (Hoshino et al., 2003; Maruyama et al., 2005; Matsuo et al., 2002). The Ruffini-like endings of whisker hairs are also absent in TrkB mutants. In addition, the number of whisker Ruffini-like endings is greatly reduced in BDNF mutants but is unaffected in NT4 mutants.
Furthermore, the number of Ruffini-like endings associated with whiskers is increased in NT3 mutantmice(Fundin et al., 1997).